Interrelationships between the renin angiotensin system and uteroplacental blood flow--a recent perspective

In pregnancy, the maternal circulating renin-angiotensin system (RAS) and uteroplacental tissue RAS has been thought to support maternal placental flow by raising maternal arterial pressure or changing placental vascular resistance. Also, the placenta or uterus may alter maternal circulating RAS. Recent studies in the authors' laboratory using chronically catheterized rabbits are compared with previous studies on interactions between the RAS and uteroplacental flow. When uterine driving pressure was reduced either mechanically or after converting enzyme inhibition, maternal placental flow decreased in proportion to change in driving pressure; myoendometrial flow did not change. Angiotensin II (AII) infusion to increase pressure by 21 +/- 2 mm Hg decreased placental but not myoendometrial flow. Thus, there is no evidence that maternal placental flow is autoregulated or supported by a specific renin-angiotensin mechanism. Normally, there is no net uterine release or uptake of active plasma renin activity, AI, or AII, but there is a small net release of trypsin-activated plasma renin activity (tPRA), presumably prorenin. Distal aortic occluder inflation produced upper-body hypertension, and uterine release of tPRA increased. There was a significant uterine arteriovenous concentration difference for AII during AII infusion. These methods are adaptable for studying interactions between uteroplacental flow and other vasoactive agents.